Historically, local area computer networks (LANs) using optical data links have relied on light emitting diode (LED) sources launching into multimode optical fibers. The EIA/TIA and IEC Building Wiring Standards (TIA 568A) specified the use of 62.5/125 micron multimode optical fiber for intra-building wiring. These standards have resulted in the large-scale deployment of multimode optical fiber in existing computer networks.
In prior communication application technologies, these data transmission platforms have provided adequate bandwidth. Asynchronous transfer mode (ATM) computer networks can support data transmission rates as high as 622 megabits/sec (MBPS), but LED rise times, the chromatic dispersion associated with the relatively wide bandwidth of light produced by the LEDs, and multiple fiber transmission modes impose an upper cap on the potential data rates. Thus, LED/multimode fiber systems are generally limited to sub-gigabit/second (GBPS) data rates.
Newer computer applications requiring higher bandwidths and the increasing number of users that must be serviced by individual networks have led the push to provide GBPS performance, and better. In order to attain this performance in the context of existing optical data links, the LED light sources have been replaced with single mode sources such as vertical cavity surface emitting lasers (VCSEL) and Fabry-Perot lasers. These devices can produce the necessary rise times and have the narrow spectral widths required for GBPS data transmission speeds.
Computer network links modified to use single mode laser sources, however, many times still fail to achieve the data/error rates at GBPS data rates that would be predicted solely from the laser source performance. The problem has been traced to computer links using multimode optical fiber. In many instances, a pulse-splitting phenomena is detected, which increases the bit error rates to unacceptably high levels at these speeds.
The obvious solution to this problem is to use single mode fiber with the single mode sources. While being viable for newly installed computer networks, such a solution is impractical for the installed base of multimode fiber networks since running new fibers in and between buildings represents a significant expense.
Other solutions have been proposed to constrain pulse splitting in signals from single mode sources that have been launched into multimode fibers. In one case, the signal from the single mode source is launched into a short-length pigtail of single mode fiber. The other end of this fiber is then coupled to the existing multimode fiber, offset from the multimode fiber core center.